How does Sacred Shield stack? (holy paladins)

I know two sacred shields will not stack, but how is the absorb shield proc decided? We have a prot pally tank and a holy paladin that use sacred shield on the tank. The holy paladin has 4-set bonus; does the shield now proc every 4 seconds? Or only if the holy paladin's shield "activate" ?In general, if X and Y cast sacred shield, do the first or last caster get prio?